Comparison of sphenoidal, foramen ovale and anterior temporal placements for detecting interictal epileptiform discharges in presurgical assessment for temporal lobe epilepsy.

OBJECTIVES: Some authors have recently stressed that the position of the tip of the sphenoidal electrode plays a crucial role in its efficacy to detect mesio-basal spikes. We have tested this hypothesis by comparing the sensitivity of a contact of a foramen ovale bundle located at the foramen ovale (CFO) with scalp electrodes in detecting interictal epileptiform discharges. We have also compared deep and superficial foramen ovale contacts in the same bundle in order to establish whether deeper contacts can detect epileptiform discharges not seen at the foramen ovale or on the scalp.
METHODS: The sensitivity for detecting epileptiform discharges of simultaneous intracranial and scalp EEG recordings from 20 patients under telemetric presurgical assessment for temporal lobe epilepsy were compared.
RESULTS: Out of 2280 epileptiform discharges evaluated, about 70% were seen only at the deep foramen ovale contacts. Out of the 722 discharges recorded by CFO and/or scalp electrodes, 698 were seen at the CFO and 690 at the scalp anterior temporal electrode. Only on 29 occasions (4.15%) were discharges recorded at the CFO and not at the anterior temporal electrode. On 21 occasions (3.04%) CFO failed to detect discharges seen at the anterior temporal electrode.
CONCLUSIONS: Our findings confirm previous results suggesting that sphenoidal electrodes, however accurately positioned, offer no significant increase in detection sensitivity compared with anterior temporal scalp electrodes. In addition, these results confirm that a large proportion of discharges seen at the deepest foramen ovale contacts are not seen either on the scalp nor at the superficial foramen ovale contacts.